fn modify_s(s: &mut String) {
    s.push_str(", world");
}


fn main() {
    let mut s1 = String::from("hello");
    modify_s(&mut s1);




    println!("Hello, world!");
}

// 悬垂引用
//fn dangle() -> &String {
//    let s = String::from("Are you ok?");
//    &s
//}
